1. Field of Invention
The present invention relates to a light guide plate, surface light source device of side light type and liquid crystal display. More specifically, the invention relates to a light guide plate having a minor face to provide a side end face through which light is inputted and having a major face through which light is outputted, and to a surface light source device of side light type and liquid crystal display which employ the light guide plate.
2. Related Art
It is known to apply a surface light source device of side light type to devices such as liquid crystal display in an backlighting arrangement of liquid crystal display panel. This arrangement is suitable for giving a thin whole structure to the device.
In general, the surface light source device of side light type is provided with a primary light source such as a rod-shaped light source which is arranged beside a guide plate formed of, for example, a transparent resin light guide plate. Illumination light emitted from the primary light source is introduced into the guide plate through a side end face (incidence face) provided by a minor face of the light guide plate.
The guide plate guides the introduced light within the plate and, in the guiding process, ion, causes a major face (emission face) to emit the illumination light. The emitted light is supplied to an object to be illuminated such as liquid crystal display panel through a prism sheet, light diffusion sheet or the like arranged additionally as required.
Two types of employable guide plates are known. Guide plates of one type have a thickness substantially uniform overall. Guide plates of the other type have a thickness tending to reduce with increasing distance from a primary light source. In general, the latter are capable of emitting illumination light more effectively in comparison with the former.
According to a prior art, an emission face or, in some cases, a back face, of a light guide plate provides an xe2x80x9cemission promotion facexe2x80x9d for promoting emission from the emission face. The prior art enables the emission face to give an increase in emission intensity. Known techniques for obtaining an emission promotion face are as follows.
(1) Utilizing of so-called light scattering pattern; According to this, a great number of locally roughened fine regions (light scattering elements) are formed on a major face of a light guide plate by (a) mat-processing, (b) etching application to an inner surface of mold to be employed in molding of the light guide plate or (c) electric discharge machining. The fine regions are distributed according to a xe2x80x9clight scattering patternxe2x80x9d. The individual fine regions have shapes such as circular dots. Through varying factors such as size or arranging pitch of individual light scattering elements, occupation area of the pattern per unit area (covering density) can be controlled.
And it is also known to flatten emission intensity (brightness of the emission face) by varying covering density depending on position in the major face (emission face).
(2) Overall roughening of major face; According to this, a major face (emission promotion face) is roughened overall, instead of forming light scattering pattern composed of a great number of locally roughened fine regions. In this case, intensity (brightness of the emission face) can be flattened, according to a prior art, by varying degree of roughening depending on position in the emission promotion face.
However, such prior techniques are subject to a problem. That is, so-called xe2x80x9creflective appearance of edgesxe2x80x9d are insufficiently avoided. As known very well, xe2x80x9creflective appearance of edgesxe2x80x9d is a phenomenon that causes an emission face to show a local emission of light which comes via an edge where an incidence face meets a major face (emission face of back face). Such local emission produces line-like excessive brightness zones (bright lines) in the vicinity of the incidence face.
Needless to say, such a phenomenon is undesirable and leads to a remarkable reduction in illumination output quality. If applied to lighting of a liquid crystal display panel, displaying quality will be greatly deteriorated.
Nevertheless the above techniques (1), (2) are able to reduce uneven brightness (bright lines) caused by such reflective appearance to some extent, a problem remains unsolved as follows.
First, if the above technique (1) (light scattering pattern) is applied, a specular surface portion (smooth surface) having no scattering ability remains among the fine light scattering elements composing the pattern, with the result that bright lines are subject to being observed through the portion.
Alternatively, if the above technique (2) (overall roughening) is applied, reflective appearance will be achieved, nevertheless adjusting range of emission promoting ability is hardly wide. For example, a great reduction in brightness tends to be caused on the emission face in the vicinities of the right and left ends of the incidence face due to being near to both ends (electrode portions) of a fluorescent lamp, and accordingly much greater emission promoting ability is required to compensate such a great brightness reduction as compared with the other part. The above technique (2) is, however, scarcely able to give actually such a great difference in emission promoting ability.
An object of the present invention is to provide a light guide plate which is improved so that the above problems are overcome. Another object of the present invention is to provide a surface light source device of side light type free from uneven brightness by the use of the improved light guide plate. Still another object of the present invention is to provide a liquid crystal display capable of avoiding reduction in displaying quality by the use of the surface light source device of side light type.
The present invention solves the above problems by employing a light guide plate having a major face on which tow kinds of light scattering patterns are formed.
First, the present invention is applied to a light guide plate comprising an incidence face provided by a minor face for light input, an emission face for light output and a back face provided by major faces, respectively.
According to a feature of the present invention, said major faces include an emission promotion face on which first emission promotion regions and second emission promotion regions are intermingled according to a distribution.
Said second emission promotion regions have an emission promotion ability per unit area smaller than that of said first emission promotion regions. And, said distribution is set so that covering density of said first emission promotion regions and that of said second emission promotion regions vary depending on position in said emission promotion face.
This enables not only reflective appearance to be prevented but also emission promoting ability to be adjusted in a wide range.
It is preferable that said second emission promotion regions are arranged among said first emission promotion regions without blank. In a typical embodiment, said covering density of said second emission promotion regions tends to decrease according to distance from said incidence face.
The present invention can be applied to a surface light source device of side light type including a primary light source and a light guide plate. The guide plate comprises an incidence face provided by a minor face, an emission face for light output and a back face provided by major faces, respectively, and the plate being arranged so that said light guide plate is supplied with light by said primary light source through said incidence face to cause said emission face to give an illumination output.
According to a feature of the present invention, said major faces include an emission promotion face on which first emission promotion regions and second emission promotion regions are intermingled according to a distribution.
Said second emission promotion regions have an emission promotion ability per unit area smaller than that of said first emission promotion regions. And, said distribution is set so that covering density of said first emission promotion regions and that of said second emission promotion regions vary depending on position in said emission promotion face.
This enables the surface light source device of side light type not only to avoid reflective appearance but also to allow emission promoting ability to be adjusted in a wide range.
Further, the present invention is also applicable to a liquid crystal display including a liquid crystal display panel and a surface light source device. According to a feature of the present invention, the above improved surface light source device of side light type is arranged for supplying illumination light. It is noted that advantages of the surface light source device to which the present invention is applied are reflected onto the liquid crystal display and, needless to say, an improved evenness in brightness of displaying image is achieved.
The above-described and other features will be understood easily from the following detailed description with referring to the attached drawings. It is should be noted that sizes of elements are partially exaggerated as required for the sake of easy understanding in the drawings.